Rfid interrogator device

ABSTRACT

An RFID interrogator device has a transmission section for transmitting a command to an RFID tag and a reception section for receiving an RF signal from the RFID tag and is configured to perform backscatter radio communication with the RFID tag. The RFID interrogator device comprises a time window setting section configured to set a time window at timing of receiving preamble data added to a head of response data transmitted from the RFID tag in response to the command, and an identifying data storage section storing preamble identifying data. The RFID interrogator device compares the data received within the time window, with preamble identifying data stored in the identifying data storage section, thereby determining whether the data received is identical to the preamble data transmitted from the RFID tag.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/879,971 filed Jul. 19, 2007, which claims priority to Japanese PatentApplication No. 2006-208834, filed Jul. 31, 2006, the entire contents ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an RFID interrogator device thatperforms backscatter radio communication with RFID tags by usingabsorption and reflection of radio waves.

2. Description of the Related Art

Any RFID tag transmits, at a predetermined bit rate, a signal composedof a sync part, a response data part and an error detecting part. Thesync part contains preamble data. U.S. Pat. No. 6,501,807 discloses anRFID interrogator device which compares preamble data preset in thedevice, with the preamble data of a signal the device has received andwhich determines that the signal received is a signal from an RFID tagif the preamble data of the signal is identical to the preset preambledata.

In the conventional RFID interrogator device, the preset preamble datais compared with the preamble data of any signal received. The signalthe device has received inevitably contains noise. A part of the noisemay be identical to the preset preamble data. In this case, the signalreceived will be mistaken for a signal from an RFID tag.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide an RFID interrogationdevice that can prevent the detection of erroneous preamble data, due tonoise, thereby increasing the precision of recognizing RFID tags.

According to an aspect of the present invention, there is provided anRFID interrogation device that has a reader function and performsbackscatter radio communication with RFID tags by using absorption andreflection of radio waves. The RFID interrogation device comprises: atransmission section configured to transmit a command to the RFID tag; areception section configured to receive an RF signal from the RFID tag;a time window setting section configured to set a time window at timingof receiving preamble data added to a head of response data transmittedfrom the RFID tag in response to the command: and an identifying datastorage section storing preamble identifying data. The RFID interrogatordevice compares the data received within the time window, with preambleidentifying data stored in the identifying data storage section, therebydetermining whether the data received is identical to the preamble datatransmitted from the RFID tag.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the general description given above and the detaileddescription of the embodiments given below, serve to explain theprinciples of the invention.

FIG. 1 is a block diagram showing the configuration of an RFIDinterrogator device according to a first embodiment of the presentinvention;

FIG. 2 is a block diagram showing the I-signal preamble detectingsection and the decoding section, both provided in the first embodiment;

FIG. 3A is a diagram showing a pattern that the preamble data may haveif the identifying data used is 12-bit data in the first embodiment;

FIG. 3B is a diagram showing the 12-bit identifying data as applied tothe preamble data shown in FIG. 3A in the first embodiment;

FIG. 3C is a diagram showing a pattern that the preamble data may haveif it is set within a time window in the first embodiment;

FIG. 3D is a diagram showing a pattern that data received and containingnoise may have in the first embodiment;

FIG. 3E is a diagram showing a pattern that the data received may haveif no time window is set in the first embodiment;

FIG. 4A is a diagram showing a pattern that the preamble data may haveif the identifying data used is 18-bit data in the first embodiment;

FIG. 4B is a diagram showing the 18-bit identifying data as applied tothe preamble data shown in FIG. 4A in the first embodiment;

FIG. 4C is a diagram showing a pattern that 18-bit preamble data mayhave if it is set within a time window in the first embodiment;

FIG. 5 is a chart illustrating the data communication between an RFIDtag and the RFID interrogator device according to the first embodiment,with respect to the time windows TW that have been preset; and

FIG. 6 is a flowchart explaining the preamble detecting processperformed in the control section provided in a second embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

FIG. 1 is a block diagram showing the configuration of an RFIDinterrogator device that includes an orthogonal demodulator. The RFIDinterrogator device comprises a control section 1, a transmissionsection 2, a reception section 3, a received data processing section 4,an external interface section 5, a circulator 6, a low-pass filter (LPF)7, and an antenna 8. The RFID interrogator device receives data from,and transmits data to, an external host apparatus via the externalinterface section 5.

The transmission section 2 is composed of an encoding section 21, anamplitude modulator 22, a phase-locked loop device (PLL) 23, a band-passfilter 24, and a power amplifier 25.

The transmission section 2 receives a transmission signal from thecontrol section 1. In the transmission section 2, this signal is inputto the encoding section 21. The encoding section 21 encodes thetransmission signal output from the control section 1.

The encoding section 21 encodes the transmission signal into, forexample, a Manchester code or an FM0 code. A Manchester code is acquiredby an encoding system, wherein data rises at the center of the bit if itis 0 and falls at the center of the bit if it is 1. In other words, thecode changes from 0 to 1 if the data is 0, and from 1 to 0 when the datais 1. An FM0 code is acquired by an encoding system, wherein the code isinverted at every bit border and even at the center of the bit if thedata is 0.

The PLL 23 supplies a local carrier signal to the amplitude modulator22. The amplitude modulator 22 modulates the amplitude of the localcarrier signal with the transmission signal supplied from the encodingsection 21. The band-pass filter 24 filters out unnecessary frequencycomponents from the transmission signal whose amplitude has beenmodulated by the amplitude modulator 22. The power amplifier 25amplifies the transmission signal that has passed through the band-passfilter 24.

The transmission section 2 is connected to the circulator 6. The signalamplified in the power amplifier 25 is supplied from the circulator 6 tothe antenna 8 via the low-pass filter 7. The antenna 8 radiates thesignal in the form of radio waves.

The reception section 3 is connected to the circulator 6. The receptionsection 3 is composed of first and second mixers 31 and 32, two low-passfilters 33 and 34, two binary coding circuits 35 and 36, a 90° phaseshifter 37, and the above-mentioned PLL 23.

The reception section 3 processes a reception signal by a so-calleddirect conversion system, in which the carrier component is removeddirectly from the reception signal.

Any RF signal the antenna 8 has received from the RFID tag is suppliedfrom the antenna 8 to the circulator 6 via the low-pass filter 7. The RFsignal is then supplied from the circulator 6 to the reception section3. In the reception section 3, the signal coming from the circulator 6is supplied to the first mixer 31 and the second mixer 32.

The first mixer 31 receives a local carrier signal from the PLL 23. Thesecond mixer 32 receives a signal supplied from the PLL 23 and having aphase shifted by 90° by the 90° phase shifter 37.

The first mixer 31 mixes the reception signal and the local carriersignal, generating an in-phase signal (I-signal) that has a componentmatching in phase with the local carrier signal. The second mixer 32mixes the reception signal and the signal obtained by phase shifting thelocal carrier signal by 90°, generating a quadrature-phase signal(Q-signal) that has a component orthogonal to the local carrier signal.

The low-pass filter 33 receives an I-signal from the first mixer 31,filters out unnecessary high-frequency components from the I-signal, andoutputs encoded data. The low-pass filter 34 receives a Q-signal fromthe second mixer 32, filters out unnecessary high-frequency componentsfrom the Q-signal, and outputs encoded data. The binary coding circuits35 converts the I-signal coming from the low-pass filter 33, into abinary signal. The binary coding circuits 36 converts the Q-signalcoming from the low-pass filter 34, into a binary signal.

The received data processing section 4 has a sync clock generatingsection 411, a time window setting section 412, a preamble detectingsection 413, a decoding section 414, and a response data error detectingsection 415, all dedicated to the I-signal. The received data processingsection 4 further has a sync clock generating section 421, a time windowsetting section 422, a preamble detecting section 423, a decodingsection 424, and a response data error detecting section 425, alldedicated to the Q-signal.

The I-signal generated by the binary coding circuits 35 is supplied fromthe reception section 3 to the sync clock generating section 411,preamble detecting section 413, decoding section 414 and response dataerror detecting section 415. The Q-signal generated by the binary codingcircuits 36 is supplied from the reception section 3 to the sync clockgenerating section 421, preamble detecting section 423, decoding section424 and response data error detecting section 425.

The sync clock generating section 411 dedicated to the I-signalgenerates, at all times, a clock signal that is synchronous with thebinary signal coming from the binary coding circuit 35. The clock signalis supplied to the control section 1, preamble detecting section 413,decoding section 414 and response data error detecting section 415. Thesync clock generating section 421 dedicated to the Q-signal generates,at all times, a clock signal that is synchronous with the binary signalcoming from the binary coding circuit 36. This clock signal is suppliedto the control section 1, preamble detecting section 423, decodingsection 424 and response data error detecting section 425.

The time window setting section 412 dedicated to the I-signal sets atime window at the time the preamble detecting section 413 acquires thepreamble data of the I-signal. The time window setting section 422dedicated to the Q-signal sets a time window at the time the preambledetecting section 423 acquires the preamble data of the Q-signal.

The preamble detecting section 413 dedicated to the I-signal comparesthe preamble data existing at the head of the I-signal, with thepreamble identifying data preset within the time window set by the timewindow setting section 412, thereby detecting the preamble data includedin the I-signal. The preamble detecting section 423 dedicated to theQ-signal compares the preamble data existing at the head of theQ-signal, with the preamble identifying data preset within the timewindow set by the time window setting section 422, thereby detecting thepreamble data included in the Q-signal. On detecting the preamble dataincluded in the I-signal, the preamble detecting section 413 outputs adetection signal to the control section 1. On detecting the preambledata included in the Q-signal, the preamble detecting section 423outputs a detection signal to the control section 1.

FIG. 2 is a block diagram that shows a reception system that detects anddecodes the preamble data. The reception system shown in FIG. 2 isconfigured to receive the I-signal. The reception system for receivingthe Q-signal has the same configuration as the reception system shown inFIG. 2.

The sync clock generating section 411 has a digital PLL circuit 4111.The sync clock generating section 411 generates a clock signal that issynchronous with the I-signal, which is a binary signal input from thebinary coding circuit 35.

An RFID tag has response data and preamble data attached to the head ofthe response data. The preamble data is of such a pattern that itchanges every 0.5 T, which is half the cycle T that corresponds to thetransmission rate of the RFID tag. Therefore, the digital PLL circuit4111 generates a clock signal whose cycle is 0.5 T, i.e., half the cycleT corresponding to the transmission rate of the RFID tag.

The sync clock generating section 411 supplies from the clock signalgenerated by the digital PLL circuit 4111 to the preamble detectingsection 413 and the decoding section 414.

The preamble detecting section 413 is composed of an identifying datastorage section 4131, a shift register 4132, and a comparator 4133, alldedicated to preamble data. The data storage section 4131 storespreamble identifying data that is used to set preambles. The comparator4133 is provided as decision means. The shift register 4132 acquires theI-signal, i.e., the binary signal input from the binary coding circuit35, in synchronism with the clock signal supplied from the digital PLLcircuit 4111. The comparator 4133 compares the bit data acquired in theshift register 4132 within the time window set by the time windowsetting section 412, with the above-mentioned preamble identifying data,thereby determining whether preamble data exists or not.

The decoding section 414 is composed of a frequency halving circuit4141, a two-input exclusive OR circuit 4142, a D-type flip-flop 4143, ashift register 4144, a counter 4145, and a data register 4146. Theexclusive OR circuit 4142 has an inverting output terminal.

The frequency halving circuit 4141 receives a clock signal supplied fromthe digital PLL circuit 4111 and having a cycle of 0.5 T. The circuit4141 then divides the frequency of this clock signal by 2, generating aclock signal having cycle T. The exclusive OR circuit 4142 extracts thebit data shifted in the shift register 4132, in units of two bits, thusgenerating an exclusive logic sum of the signals input to it. The D-typeflip-flop 4143 supplies the output of the exclusive OR circuit 4142 tothe D input terminal. The flip-flop 4143 also supplies the clock signalcoming from the frequency halving circuit 4141, to the CLK terminal.Having this circuit configuration, the D-type flip-flop 4143 decodesevery two bits stored in the shift register 4132 into “1” if the bitsare [0,0] or [1,1], and into “0” if the bits are [1,0] or [0,1].

The data decoded is supplied from the D-type flip-flop 4143 to the shiftregister 4144. The counter 4145 counts the digits constituting the datadecoded. The data register 4146 acquires this data every time data of apredetermined length is input to the shift register 4144. The data thusacquired is output to the control section 1.

The relation between the preamble pattern, the preamble identifying dataand the time window will be described, with reference to FIGS. 3A to 3E.

FIG. 3A shows a pattern that preamble data D1 may have. The preambledata D1 is 20-bit data, “10101010110100100011.”

The data storage section 4131, which is provided to store preambleidentifying data, stores the lower 12 bits of the preamble data D1,i.e., “110100100011.” These 12 bits, or identifying data P1, will beshown as black dots in FIG. 3B, if they are applied to the preamble datashown in FIG. 3A.

The broken lines shown in FIG. 3B indicate a time window TW1 that thetime window setting section 412 sets for the preamble data D1. As seenfrom FIG. 3B, the pattern corresponding to the identifying data P1exists within the time window TW1.

The higher bits of the preamble data D1 are unstable as the signalreceived by the binary coding circuit 35 rises. This is why the lowerbits of the preamble data D1 are used as identifying data P1.

The binary signal corresponding to the I-signal extracted from thesignal coming from the RFID tag is input to the shift register 4132 ofthe preamble detecting section 413. The shift register 4132 receives thebinary signal, while shifting the signal bit by bit. At the time theshift register 4132 receives the first twenty bits, the comparator 4133compares the data in the shift register 4132 with the identifying dataP1, within the time window TW1 set by the time window setting section412. More precisely, the comparator 4133 compares the lower twelve bitsin the shift register 4132 with the identifying data P1. If these bitsare identical to the bits constituting the comparator 4133, it isdetermined that the signal received is the preamble data D1. The datafollowing the preamble data D1 is then received as response data fromthe RFID tag.

FIG. 3C shows a pattern that the preamble data may have if receivedwithin the time window TW1. As white dots indicate in FIG. 3C, dataidentical to the identifying data P1 exists within the time window TW1.Therefore, the preamble data coming from the RFID tag is correctlydetected in this case.

FIG. 3D shows a pattern of the preamble data received and containingnoise. In this case, the pattern existing within the time window TW1 isdifferent from the pattern that corresponds to the identifying data P1.The preamble data is therefore determined to be erroneous.

FIG. 3E shows a pattern that the signal received may have when the timewindow TW1 is not set. This signal contains noise. The white dots shownin FIG. 3E coincide with the identifying data P1. These white dots arenot detected as preamble data, since the time window TW1 is not set.Consequently, the signal that follows the white dots is not detected asresponse data coming from the REID tag, either.

How the preamble data of the I-signal is detected has been explainedabove. The preamble data of the Q-signal is detected in the same manner.

The signal received may be inverted, depending on its phase. It istherefore desired that the lower twelve bits in the shift register 4132be determined to be preamble data if they are identical to the invertedpattern of the identifying data P1, or P1′=“001011011100.”

As mentioned above, the lower twelve bits are used as identifying data.Nevertheless, the identifying data may be constituted by any other bits.

FIG. 4A to 4C show the case where the lower eighteen bits of thepreamble data are used as identifying data. That is, the lower eighteenbits of the preamble data D1 having the pattern shown in FIG. 4A, i.e.,“101010110100100011,” are used as identifying data P2.

The pattern of this identifying data P2 will be shown as black dots inFIG. 4B, if they are applied to the preamble data D1 shown in FIG. 4A.In this case, the time window TW2 set for the preamble data D1 by thetime window setting section 412 is broader than the time window TW1 asindicated by broken lines in FIG. 4B. That is, the width of the timewindow varies, depending on the number of bits used to determine whetherthe preamble is identical to the preamble identifying data.

FIG. 4C shows a pattern the preamble data may have if it is receivedwithin the time window TW2. In this case, data identical to theidentifying data P2 (=“101010110100100011”) exits within the time windowTW2 as indicated by white dots. Hence, correct preamble data coming fromthe RFID tag can be detected.

The number of bits constituting the identifying data is thus increased,thereby further decreasing the probability of detecting anoise-containing signal, erroneously as preamble data. This can morereliably prevent erroneous detection of the preamble data.

A method of setting the time window TW in the time window settingsections 412 and 422 will be explained. More precisely, how to set atime window for the I-signal will be explained here. Note that a timewindow for the Q-signal is set in the same manner.

FIG. 5 illustrates the data communication between an RFID tag and theRFID interrogator device, with respect to the time windows TW that havebeen preset.

The present embodiment utilizes a backscatter scheme as a radiocommunication scheme. The backscatter scheme uses the absorption andreflection of radio waves transmitted from the transmission section 2 ofthe RFID interrogator device, so that the RFID interrogator device mayaccomplish radio communication with the RFID tag.

When the RFID interrogator device transmits a Query command to the RFIDtag, the RFID tag responds to this command. In period A, the RFID tagcorrectly responds to the command, sending an appropriate response tothe RFID interrogator device. In period B, a plurality of RFID tagsrespond at the same time, causing collision of responses.

The response time T1′ the RFID tag has to the Query command is known,and the fluctuation of the response time T1′ is known, too. The responsetime T1′ is given as follows:

T1′MIN<T1′<T1′MAX  (1)

In the case of, for example, EPC Global, Class 1, Generation 2, which isnow virtually a global standard, the minimum value T1′MIN for responsetime T1′ is 238 vec, and the maximum value T1′MAX for response time T1′is 262 vec, if the transmission rate is 40 kbps.

The preamble detecting section 413 has a delay time TD1 for thetransmission system and a delay time TD2 for the reception system, bothresulting from the processing of digital signals. The delay time TD1 andthe delay time TD2 are known because they are design values. Hence, thetime T1 that the preamble detecting section 413 requires to receive thepreamble data from the RFID tag after finishing the transmission of theQuery command is as follows:

T1=TD1+T1′MIN+TD2  (2)

The time window TW is determined as follows:

TW=0.5 T×N+(T1′MAX−T1′MIN)  (3)

where N is the number of bits constituting the preamble identifying dataused when the time window TW is applied, and 0.5 T is the samplingcycle.

That is, the time based on the minimum value T1′MIN and maximum valueT1′MAX for the response time preset for the RFID tag has been added tothe time window TW. More specifically, the time equivalent to thedifference between the maximum and minimum values T1′MAX and T1′MIN forthe response time is added to the time window TW.

Therefore, the time window TW1 is given as follows, if the lower twelvebits of the preamble data D1 shown in FIG. 3A (that is, N=12) are usedas identifying data P1:

TW1=0.5 T×12+(T1′MAX−T1′MIN)

The time window TW2 for the case where the lower eighteen bits of thepreamble data D2 shown in FIG. 4A (that is, N=18) are used asidentifying data P2 is given as follows:

TW1=0.5 T×18+(T1′MAX−T1′MIN)

The time window TW need not absolutely accord with the equation (3). Itmay be larger than is defined in the equation (3). If it is excessivelylarge, however, the probability of detecting noise as preamble data willincrease. It is therefore undesirable to expand more than necessary.

The time window TW is opened upon lapse of the sum of time T1 and timet1 after the RFID interrogator device has transmitted the Query command.Time T1 is the time the device needs in order to receive the preambledata from the RFID tag. Time t1 is obtained by multiplying the samplingcycle 0.5 T by the difference between the number of preamble data bitsand the number of preamble identifying data bits (N). If the preambledata consists of 20 bits, time t1 is:

t1=(20−N)×0.5 T

The time at which the RFID interrogator device detects the preamble datais the sum of the time (T1+t1) and the time window TW. That is, thedevice detects the preamble data upon lapse of (T1+t1+TW) after it hastransmitted the Query command.

In period A, the RFID interrogator device detects the preamble datacoming from the RFID tag upon lapse of (T1+t1+TW) after the transmissionof the Query command, and then outputs a preamble detection signal tothe control section 1. In period B, the signals from a plurality of RFIDtags collide with one another, generating noise, and the preamble datais determined to be erroneous in the time window TW.

In the RFID interrogator device so configured as described above, thecontrol section 1 inputs the Query command to the transmission section2. The transmission section 2 supplies the Query command via thecirculator 6 and low-pass filter 7 to the antenna 8. The antenna 8transmits the Query command. There is a time delay TD1 between the timethe control section 1 inputs the Query command and the time the antenna8 transmits the Query command.

If any RFID tag should respond to the RFID interrogator device at thistime, it receives the Query command coming from the RFID interrogatordevice. Upon lapse of time T1′, the RFID tag transmits response data tothe RFID interrogator device.

When the RFID interrogator device receives the response data from theRFID tag at the antenna 8, the reception section 3 receives the responsedata via the low-pass filter 7 and circulator 6. In the receptionsection 3, the response data is input to the first and second mixers 31and 32.

The first mixer 31 outputs an I-signal. The I-signal is supplied via thelow-pass filter 33 to the binary coding circuit 35. The binary codingcircuit 35 converts the I-signal to a binary I-signal. The binaryI-signal is input to the sync clock generating section 411, preambledetecting section 413, decoding section 414 and response data errordetecting section 415, all dedicated to the I-signal.

The second mixer 32 outputs a Q-signal. The Q-signal is supplied via thelow-pass filter 34 to the binary coding circuit 36. The binary codingcircuit 36 converts the Q-signal to a binary Q-signal. The binaryQ-signal is input to the sync clock generating section 421, preambledetecting section 423, decoding section 424 and response data errordetecting section 425, all dedicated to the Q-signal.

The preamble detecting section 413 dedicated to the I-signal detects thepreamble data added to the head of the response data received within thetime window TW set by the time window setting sections 412 that isdedicated to the I-signal. Then, the preamble detecting section 413compares the preamble data, thus detected, with the preamble identifyingdata that has been already stored.

The preamble detecting section 423 dedicated to the Q-signal detects thepreamble data added to the head of the response data received within thetime window TW set by the time window setting sections 422 that isdedicated to the Q-signal. Then, the preamble detecting section 423compares the preamble data, thus detected, with the preamble identifyingdata that has been already stored.

The time window TW starts when the RFID interrogator device finishestransmitting the Query command to the RFID tag and ends when the period(T1+t1) elapses thereafter. If the preamble identifying data is consistof the lower twelve bits of the preamble data, the lower twelve bits ofthe preamble data added to the response data received will fall withinthe time window TW.

The data existing in the time window TW are compared with the preambleidentifying data. If the response data received from the RFID tag iscorrect data, the data within the time window TW will be identical tothe preamble identifying data. In this case, the RFID interrogatordevice determines that the response data has been received from the RFIDtag.

If the response data received from the RFID tag contains noise and istherefore incorrect data, however, the data within the time window TWwill not be identical to the preamble identifying data, at highprobability. In this case, the RFID interrogator device does notdetermine that the response data has been received from the RFID tag.

Erroneous detections of the preamble data, due to noise, can thus beavoided in the present embodiment. This can enhance the precision ofrecognizing RFID tags.

Second Embodiment

A second embodiment of the present invention will be described, in whichthe received data processing section 4 used in the first embodiment isincorporated in the form of software and a program is executed todetermine whether the preamble data exists or not. The second embodimentis identical to the first embodiment, except that the received dataprocessing section 4 is incorporated in the control section 1. Thus, noblock diagrams of the second embodiment are attached hereto, and theconfiguration of the second embodiment will not be described.

The control section 1 executes the program, detecting the preamble asillustrated in the flowchart of FIG. 6. First, the control section 1sets the count of the counter n to “1” in Step S1. Then, a timer startsmeasuring time T in Step S2. In Step S3, the control section 1determines whether T≦(T1+t1+TW) or not.

If T≦(T1+t1+TW), the control section 1 stores the received data into amemory in Step S4. In Step S5, the control section 1 determines whetherthe data in the memory is identical to the preset preamble identifyingdata.

If the data is not identical to the preset preamble identifying data,the process returns to Step S3. Thereafter, Steps S3 to S5 are repeated.

The control section 1 may determine in Step S5 that the data in thememory is identical to the preset preamble identifying data. If this isthe case, the control section 1 determines in Step S6 whether T≧(T1+t1).

If T≧(T1+t1) in Step S6, the control section 1 determines that the datais identical to the preamble identifying data within the time window TWand then terminates the process of detecting the preamble. In otherwords, the data received is found to contain the preamble data comingfrom the RFID tag. The control section 1 then performs a process ofacquiring the response data that follows the preamble data. This processwill not be explained here.

Even if the data in the memory is found to be identical to the preambleidentifying data, the control section 1 determines that the preambledata has not been detected, unless T≧(T1+t1). That is, the controlsection 1 determines that the data is outside the time window TW anddoes not recognize the data as preamble data. The process then returnsto Step S3.

If the time T measured by the timer increases over (T1+t1+TW) while nopreamble data is being detected, the control section 1 determines thatthe preamble data has not been detected. In this case, the controlsection 1 determines in Step S7 whether the count of the counter n hasreached a predetermined value N. Unless n=N, the control section 1increases the count of the counter n by one in Step S8. Then, in StepS9, the control circuit 1 resets time T in the timer. The process thenreturns to Step S2. Steps S2 to S5 and Step S6 are repeated.

No preamble data may be detected even if the count of the counter nreached the value N. In this case, the control section 1 determines thatthere exists no preamble data to detect and terminates the process ofdetecting the preamble.

To detect preamble data by using software, too, a time window TW is setand it is determined whether the data in the memory is identical to thepreamble identifying data within the time window TW. Thus, no preambledata is detected at high probability if the reception signal is notcorrect response data coming from the RFID tag, but data that containsnoise. Erroneous detection of preambles, due to noise, can therefore beavoided. This can enhance the precision of recognizing RFID tags.

To detect preamble data, whether the data in the memory is identical tothe preamble identifying data within the time window TW is determinednot only once, but several times. A preamble can therefore be eventuallydetected even if it has not been detected because the response datacoming from a RFID tag contains temporary noise. Thus, the RFIDinterrogator device can reliably receive the response data transmittedfrom the RFID tag.

In the embodiments described above, the transmission section 2 and thereceived data processing section 4 are components that workindependently of each other. Nonetheless, the received data processingsection 4 may be incorporated into the transmission section 2, so that alarger transmission section may be provided.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. An RFID interrogator device which transmits a command to an RFID tagand receives response data at a sampling cycle corresponding to atransmission rate of the response data from the RFID tag, the RFIDinterrogator comprising: a first data storage section configured tostore as preamble identifying data prescribed lower bits of preambledata attached to a head of the response data; a second data storagesection configured to store received data; a timer to measure apredetermined time based on a time obtained by adding to a time requiredto receive preamble data from the RFID tag after finishing thetransmission of command to the RFID tag a time obtained by multiplyingthe sampling cycle by a difference between the number of preamble databits and the number of preamble identifying data bits; and a comparatorto compare data stored in the first data storage section and datareceived within the predetermined time and stored in the second datastorage section.
 2. The RFID interrogator device according to claim 1,wherein the predetermined time is obtained by adding to a time obtainedby multiplying the number of preamble identifying data bits by thesampling cycle a time equivalent to a difference between a maximum valueand a minimum value for a preset response time of the RFID tag.
 3. TheRFID interrogator device according to claim 1, wherein the second datastorage section is a shift register.
 4. A radio communication method fortransmitting a command to an RFID tag and receiving response data at asampling cycle corresponding to a transmission rate of the response datafrom the RFID tag, the method comprising: preparing prescribed lowerbits of preamble data attached to a head of the response data aspreamble identifying data; and comparing data received within apredetermined time and the preamble identifying data based on a timeobtained by adding to a time required to receive preamble data from theRFID tag after finishing the transmission of command to the RFID tag atime obtained by multiplying the sampling cycle by a difference betweenthe number of preamble data bits and the number of preamble identifyingdata bits.
 5. The radio communication method according to claim 4,wherein the predetermined time is obtained by adding to a time obtainedby multiplying the number of preamble identifying data bits by thesampling cycle a time equivalent to a difference between a maximum valueand a minimum value for a preset response time of the RFID tag.
 6. Theradio communication method according to claim 4, wherein the preambleidentifying data is stored in a memory.